When there are no more individuals of a species (plant or animal) alive anywhere in the world, the species is said to be extinct. Extinction is also known as the process of dying out. The process is a completely natural part of evolution. But sometimes extinctions happen at a higher and a faster rate than usual. Factors such as large body size, small population, and position at a higher trophic level favour extinction. Mass extinction of species has occurred during Permian, KT Boundary and Pleistocene period.